Abstract : Measurements of the spontaneous polarization in KH2PO4 within 1K of the ferroelectric transition are reported. The measurements employed the electrocaloric effect, allowing simultaneous determination of the polarization and its temperature derivative. The derivative computed from polarization measurements compares well with direct determinations of (delta P/delta T)E demonstrating thermodynamic consistency of the measurements. Landau theory allows separate calculation of specific heat contributions of the lattice and polarization and thus the total specific heat, which agrees very well with direct calorimetric determinations, although the decomposition differs from that previously assumed. This indicates that Landau theory provides a good description of the transition in KH2PO4 and that there is a calorimetric anomaly not directly associated with the polarization. The free energy expansion best fitting the data contains P4 and P8 terms and no P6 term. (Author)